The present invention is directed to an anchoring arrangement for a rod-shaped tension member formed of a fiber reinforced composite material. Such a tensioning member can be used as an anchoring member in foundation or mining construction, or as a reinforcing member in concrete or prestressed concrete construction.
Recently, in civil engineering construction, tension members formed of high strength fiber reinforced composite materials have found increasing use in place of steel tension members. Tension members formed of fiber reinforced composite materials, such as polyamides or aramides, reinforced by glass or carbon fibers, have a low specific gravity as compared to a comparable tensile strength steel and, in addition, such tension members have a high resistance to corrosion. Tension members, based on such properties, can be used advantageously as anchoring rods in foundation and mining construction, where they are often exposed to aggressive water action which promotes corrosion in steel tension members.
Tension members or rods formed of fiber reinforced composite materials act under load similar to steel tension members, that is, they have an essentially linear stress-strain characteristic. Contrary to steel tension rods, however, they have no yield point or elastic limit, instead, they fail upon reaching a given tensile strength without any previous plastic deformation. This characteristic and the very low lateral compression strength, as compared to their strength in the elongated direction of the fibers, has prevented the previously known anchoring arrangements from being utilized in construction.
Another problem has been noted in such anchoring arrangements. Due to the length required for force transmission between a tension rod and an anchoring member, such as an anchoring nut, the danger increases that the deformation in the tension member and in the anchoring member are no longer comparable to one another and they may even act counter to one another. As an example, an anchoring nut bearing against an anchor plate is subjected to compressive stress and, as a result, is upset, while at the same time, the tension rod is subjected to tensile stress and is elongated. In the case of sleeve or socket connections of tension rods, the connecting sleeve is also elongated, however, the elongation of the tension member and the sleeve do not coincide because of their different cross-sectional areas or at the least, the elongations are unevenly distributed along the length of the sleeve connection.
When metallic materials are used, such as steel, such a problem is solved mainly by plastic deformation of the material itself which has the property of adapting to load peaks by yielding. When non-metallic materials are used, however, these problems are very evident. It should be noted when tension members are formed of fiber reinforced composite materials, there is an elongation four times greater than experienced in steel at the same tensile stress, however, because of its very low laminar shear strength and high lateral compressive sensitivity, relatively long distances are required for mutual force transmission between the tension rod and the anchoring member.
Attempted uses of fiber reinforced composite material rods, as prestressing members, in prestressed concrete construction has resulted mainly in anchoring by frictional locking achieved by clamping forces or by bonding. Anchoring produced by frictional locking, however, is not only expensive because of the necessity to generate clamping forces for its effectiveness, there are other problems because the dependability of such an anchoring arrangement over long periods of time depends upon the materials, particularly those used for the clamping member and must maintain their properties over extended periods of time. The frictional locking action cannot be guaranteed with any certainty, since creeping due to ageing must be taken into account. If the anchoring is effected by bonding, there is the disadvantage, in order to produce an anchor at the desired point of the tension rod, that more or less extensive measures have to be taken requiring considerable expense and apparatus which must be available at the construction site.
For the use of fiber reinforced composite material rods, comparable to conventional reinforcing rods for concrete, it has been known to provide such a rod with a profiled surface for improving the bond with the concrete. The profiled surface can be provided by helically shaped grooves or ribs, such as disclosed in DE-U 19 36 073. A point type of anchoring by anchor members is not mentioned in this connection, it would lead to the problems mentioned above.
Finally, it has been known from DE 37 03 974A1, to construct a rod-shaped tension member from high strength unidirectional fibers enclosed by a jacket in a shear resistant manner. The material of the jacket has an extensibility larger than that of the fiber and is plastically deformable if overstressed. Further, it is possible to provide the jacket with a profiled surface suitable for positive locking engagement with anchoring or connecting members having a correspondingly shaped surface profile, such as in the form of a coarse thread. With this jacket, deformation compensation can take place in an anchoring region between the tension member and an anchoring body with load peaks being carried by the plastic deformation. In every instance, the anchoring force must be transmitted by the adhesive bond between the tension member and the jacket.